教甄◆英文科題庫下載題庫

       When earthquakes occur, ruptures in geological faults produce two different types of deformations—static and dynamic. Static deformations are caused by the fault transitioning from a relaxed position to a stressed one, due to plate tectonic motion, before returning back to a relaxed state, resulting in permanent changes to the geography of the terrain. Dynamic deformations, on the other hand, emanate from the rupture, moving swiftly through the earth’s rock. The bulk of the energy generated by an earthquake goes into the static deformation, with the remainder of it dissipating through dynamic deformation in the form of seismic waves. These seismic waves can be categorized into four types, two of which move below the surface—known as body waves.

       It is the surface waves that are ultimately responsible for virtually the entirety of the destruction inflicted by earthquakes despite possessing a lower frequency than body waves and moving at considerably slower speeds. The two types of surface waves are Love waves and Rayleigh waves, both of which are named for the men who provided vital mathematical calculations pertaining to them—A.E.H. Love and Lord Rayleigh. Love waves are the faster of the two, and they generate only horizontal motion. As a result, individuals experiencing a Love wave will feel the earth swaying from side to side. Rayleigh waves, on the other hand, roll across the surface of the earth much in the way ripples move across the surface of a pond, resulting in both vertical and horizontal motion. Because of this, they are by far the more destructive of the two types of surface waves.

       Body waves, high-frequency, fast-moving waves that travel beneath the surface of the earth, can be divided into primary waves, also called P waves, and secondary waves, often referred to as S waves. As their name implies, primary waves are the first to reach any given point after an earthquake occurs, as they are the fastest of all seismic waves. They are able to pass through solids, liquids and gases, and do so with a pushing forward and pulling backward motion, which closely resembles the manner in which sound waves move through the air and is the reason why they are sometimes called compression waves. The precise speed of P waves depends on the depth. They move at about 6 kilometers per second near the surface and as fast as 11 kilometers when they are closer to the earth’s core. S waves, also known as shear waves, can cause the ground to move alternately from one side to the other, as well as in an up-and-down motion. Their average speeds are slightly more than half those of P waves, and they are only capable of moving through solids.

       Despite their destructive properties, seismic waves do provide scientists with valuable data. By detecting P waves and S waves with seismographs, scientists are able to determine the epicenter of an earthquake. Although there are naturally variations in wave speeds due to geologic conditions, the ratio between the average speeds of the two types of waves remains fairly constant. This allows seismologists to time the gap between the arrival of the P wave and that of the S wave, a measurement that produces a fairly accurate calculation of the distance of the earthquake from the observation station. What’s more, the inability of S waves to move through fluids furnished geologists with the first clues that the earth has a liquid outer core, which has since been established as an important scientific fact.